Emissions of gamma rays are normally associated with high-energy astrophysics, not thunderstorms, but theoretical work going as far back as 1925 has predicted gamma-ray emissions to occur in strong thunderstorm electric fields. However, only recently, thanks to advances in electronics and thunderstorm ballooning techniques, has it become possible to measure gamma-ray emissions directly in thunderstorms. Eack et al. now report balloon measurements that reveal a greater than threefold increase in the gamma-ray flux in the anvil (or “thunderhead”) of a summer thunderstorm. The enhanced gamma-ray emissions are most likely the result of a runaway effect, in which a “seed” electron with an energy of about 1 million electron volts produced by a cosmic ray initiates an avalanche of energetic electrons. The measurements indicate that gamma ray production in thunderstorms is not confined to isolated regions near the main charge centers of large thunderstorm complexes (where previous measurements were obtained) and may thus be more widespread than previously believed.—JU